Electronic and electromechanical systems that are implanted in the body of animals, such as sensors and their associated electronic circuits that function to amplify the sensor signals and transmit them to a nearby receiver, require a power source. Currently, these systems are powered externally by batteries. The smallest batteries are, however, much larger than the implantable sensors and their associated signal amplifier circuits. For this reason, the size of autonomous packages that include a sensor, an amplifier-transmitter, and a power source are generally defined by the battery. Batteries cannot be made as small as the sensors or amplifiers because the batteries require cases and seals, the miniaturization of which is difficult and prohibitively expensive.
Known fuel cells are also much larger than available sensors because they require a case and a seal, and usually also a membrane, which is difficult to miniaturize and seal. Biological fuel cells, also known as biofuel cells, have been described in the past fifty years. However, only a few of these biofuel cells could be operated under physiological conditions. Physiological conditions include, for instance, a pH of about 7.2 to 7.4, a temperature of near 37° C., and a chloride concentration of about 0.14 M. Furthermore, known biofuel cells having higher power densities require ion-conducting separation membranes.
Accordingly, there still exists a need for the development of a system having millimeter to sub-millimeter dimensions that can function under physiological conditions to provide power to a sensor.